The building is unusual even by the standards of early Irish ecclesiastical architecture: its lintelled doorway opens from the east wall rather than the more conventional west, which in turn forces the east-facing window into an awkward position near the south-east corner. That window once had a head and rear arch, both now gone, and a thin projecting flag below it was probably the altar stone. The walls, between one and 1.25 metres thick, show no sign of corbelling, the technique of overlapping inward-leaning stones used in related structures such as the famous Gallarus Oratory on the Dingle Peninsula. Inside, a stone font and a small cresset-stone, a lamp consisting of a tapering stem and hollowed bowl for liquid fuel, were formerly kept; the lamp is now in the National Museum of Ireland.

The monastery around the oratory was never enclosed by a formal boundary wall in the conventional sense, but roughly half an acre of terraced hillside holds a graveyard, a leacht, and a holy well. A leacht is a rectangular commemorative cairn of dressed drystone masonry, and this one measures 7.45 metres long and just over a metre high, with a small lintelled chamber set into its base and a stone cross standing loosely at its northern end. Among the objects formerly associated with the site was a remarkable four-faced ogham stone, each face carrying a cross of a different design, including one face bearing a swastika-like form and another whose cross arms terminate in equilateral triangles. When the antiquary Windele first encountered it, the stone lay in front of the oratory; by 1901 it had been built in as a lintel; and in 1902 it was removed to Trinity College Dublin, where it remains. A cross-slab recorded by Macalister in 1949 bearing the inscription OR DO MACRUED Ú DALACH, an appeal for a prayer on behalf of a named individual, has since disappeared entirely. The earliest documentary reference to the island itself appears in the Register of the Hospital of St. John the Baptist in Dublin and dates to the early thirteenth century, recording a deed of gift from one Adam Dundno of a mark of silver owed as annual rent for the island by a Ricardus de Marisc. The island was uninhabited by 1756 and only intermittently occupied by one or a few families during the nineteenth and early twentieth centuries.